The present invention relates to an apparatus for measuring a fine clearance between two articles, and more particularly to a measuring apparatus for precisely measuring a spacing of a magnetic head of a magnetic disc unit.
As an example of measurement of a fine clearance, the measurement of the spacing of the magnetic head of the magnetic disc unit is explained. The magnetic head assembly floats on a surface of a magnetic disc (information record medium) rotating at a high speed while keeping a clearance (called the spacing) in the order of submicron by an aerodynamic wedge action. In order to maintain a reliability of the magnetic head assembly which maintains a small spacing in a stable manner, it is essential to detect the spacing. Since the magnetic head is mounted on a slider, a condition of the floating depends on the slider. Hereinafter, the magnetic head is referred to as the slider.
The measurement of the fine clearance (spacing) between a dummy glass disc of the magnetic disc unit and the slider by using a principle of a measuring method which utilizes a light interference method disclosed, for example, in the article "An Application of White Light Interferometry in Thin Film Measurements" by C. Lin & R. F. Sullivan, IBM J. Res. Develop., Vol. 16, No. 3 (1972), pp 269-276 is explained below.
A light from a white light source is converted by a filter into a monochromatic light flux, which is directed to a half-mirror. A portion of the light flux is directed by the half-mirror to the glass disc having an anti-reflection film coated on a surface thereof, and a portion of the incident light flux is reflected by a rear surface of the disc and the remaining portion is reflected by the surface of the slider. An interference fringe is formed by the reflection light from the surface of the slider and the reflection light from the back surface of the glass disc. The interference fringe is observed by a camera or an eye through the half-mirror.
The camera may be a Polaroid camera and a picture photographed is observed to calculated a float distance.
In the past, the positions of the interference fringe on the slider are read by direct observation or by taking a photograph to determine the spacing. This measuring method has disadvantages in that (i) it takes a large manpower and a long time, (ii) the order of the interference fringe cannot be directly determined, and (iii) precision of measurement is low. To determine the order, white light may be applied and the spacing may be determined by an interference color of the reflection light. However, this method cannot provide a high precision of measurement.
In another method, a similar interferometer is used, a spacing measurement position on the slider is previously determined by pattern position detection means, a change of an interference signal at the measurement position is detected by photo-electric conversion means while a wavelength is varied, and the float distance is determined based on the wavelength which causes a maximum or minimum electric signal. This measuring method has disadvantages in that:
(1) Since a maximum or minimum area of an interference light is spread to compare with a spot at the measuring point, it cannot be exactly related to the scanning wavelength.
(2) Since the measurement is affected by a reflection coefficient and a surface roughness of the slider, the reproducibility of the measurement is low.
(3) Since the measuring position is previously determined by the pattern position detection means, the measurement is doublefold. If the previously determined measuring position varies by a small vibration during the measurement, it is difficult to correct it. Thus, a precision of measurement is low.
In a recent magnetic disc unit, a storage capacity increases, a recording density increases, and a spacing between a magnetic head and a recording medium decreases more and more (0.3-0.45 .mu.m). Accordingly, in order to maintain a reliability, precise and accurate measurement of the spacing and reduction of manpower and time of the measurement have become important goals.